Don't underestimate the complexities of bolting - Reap the benefits of reduced bolt stiffness

MAY 2015

Gaskets, dirt, irregular/distorted member contact faces reduce member stiffness and can help promote failure by fatigue mechanisms.

Together with correct installation and pretensioning of threaded fasteners, good joint design is fundamental to prevent failure of the fastener by fatigue mechanisms. 

The proportion of the applied load carried by the bolt is dependent on the ratio of the bolt to member stiffness, Fb=Fi+CP where Fi is the preload, P is the applied load and C=Kb/(Kb+Km) where Kb and Km are the bolt and member stiffnesses respectively.  Increasing the member stiffness and decreasing the stiffness of the bolts both decrease the proportion of the applied load carried by bolts. 

Bolt stiffness is related to area and length by the equation Kb=AE/l where A is the cross sectional area, E is the young's modulus and l is the grip length of the bolt.   Although both decreasing the area of the bolt and increasing the length would both decrease the bolts stiffness, hence reducing the proportion to the applied load, decreasing the area would decrease the load that can be carried by the bolt.  Increasing the length of the bolt by including a stiff thick washer or collar (i.e. a long cylinder) decreases the stiffness of the bolt without decreasing the axial load carrying capacity of the joint.  Collars are regularly used to good effect in high load applications such as crane bases. 

These gains are relatively easy to achieve - where possible make every effort to decrease bolt stiffness and increase member stiffness to prevent premature bolt failure!! 

Published in Technical Tips by Origen Engineering Solutions on 1 May 2015